The present disclosure relates to a duplex stainless steel sheet having a high nitrogen content, and, more particularly, to a method for manufacturing a duplex stainless steel sheet having a high nitrogen content and good surface quality through a twin roll casting process.
In general, a twin roll strip casting process refers to a process of directly and continuously producing a strip having a thickness of several millimeters (mm) from molten steel supplied between a pair of rotating casting rolls. Referring to FIG. 1, a twin roll strip caster 100 for twin roll strip casting generally includes casting rolls 110, a ladle 120, a tundish 130, a casting nozzle 140, a meniscus shield 150, brush rolls 160, and edge dams 170.
In a twin roll strip casting process, molten steel is supplied to the ladle 120, and the molten steel flows to the tundish 130 through a nozzle. Then, the molten steel is supplied from the tundish 130 to a region among the casting rolls 110 and the edge dams 170 attached to both ends of the casting rolls 110 through the casting nozzle 140, and the molten steel starts to solidify in the region. At this time, the meniscus shield 150 protects the surface of the molten steel solidifying in the region between the casting rolls 110 so as to prevent oxidation, and a proper gas is supplied to control the atmosphere of the region. In this state, while the molten steel solidifies, the molten steel is drawn from the region through a gap between the casting rolls 110 as a strip 180.
However, when a high nitrogen duplex stainless steel sheet is manufactured through such a twin roll casting process, the solubility of nitrogen in molten steel varies as the molten steel solidifies, and thus nitrogen gas is generated from the molten steel. This may cause surface swelling as illustrated in FIG. 2, in which an image of a duplex stainless steel sheet is illustrated.
The duplex stainless steel sheet having surface swelling illustrated in FIG. 2 may have central cracks in the swelling region as illustrated in a cross-sectional view of FIG. 3A. In addition, as illustrated in FIG. 3B, pores may be densely formed in a middle portion of the duplex stainless steel sheet in a region in which the duplex stainless steel sheet swells slightly. Thus, it may be understood that such swelling occurs when nitrogen gas is generated and expanded in a middle portion of a steel sheet.
Nitrogen has a relatively high solubility on the level of 2500 ppm to 3500 ppm in molten high nitrogen duplex stainless steel, but has a relatively low solubility on the level of 1200 ppm in solid high nitrogen duplex stainless steel. Therefore, if molten duplex stainless steel having a nitrogen content of 2500 ppm is slowly solidified, nitrogen gas may be generated in an amount corresponding to the solubility difference, 1300 ppm, and many pores may be formed in the duplex stainless steel as the nitrogen gas expands.
Due to these reasons, it is difficult to perform a general continuous casting process or ingot casting process on high nitrogen duplex stainless steel. However, in a twin roll strip casting process, the generation of bubbles may be reduced by rapid cooling, and thus a twin roll strip casting method may be applied to high nitrogen duplex stainless steel. However, even in a twin roll strip casting process, if the content of nitrogen in molten steel is high, surface swelling may occur due to the generation of nitrogen gas as illustrated in FIGS. 2, 3A, and 3B. Therefore, techniques for solving these problems are necessary.